In certain circumstances, such as in oxygen-blown gasification-gas turbine power generation processes (e.g., coal plus oxygen derived fuel gas feeding the humidified air turbine cycle or the gas turbine-steam turbine combined cycle) or in processes for steel making by the direct reduction of iron ore (e.g., the COREX.TM. process) where the export gas is used for power generation, both oxygen and pressurized nitrogen products are required. This need for pressurized products makes it beneficial to run the air separation unit which produces the nitrogen and oxygen at an elevated pressure. At elevated operating pressures of the air separation unit, the sizes of heat exchangers, pipelines and the volumetric flows of the vapor fraction decrease, which together significantly reduces the capital cost of the air separation unit. This elevated operating pressure also reduces the power loss due to pressure drops in heat exchangers, pipelines and distillation columns, and brings the operating conditions inside the distillation column closer to equilibrium, so that the air separation unit is more power efficient. Since gasification-gas turbine and direct steel making processes are large oxygen consumers and large nitrogen consumers when the air separation unit is integrated into the base process, better process cycles suitable for elevated pressure operation are required. Numerous processes which are known in the art have been offered as a solution to this requirement, among these are the following.
U.S. Pat. No. 3,210,951 discloses a dual reboiler process cycle in which a fraction of the feed air is condensed to provide reboil for the low pressure column bottom. The condensed feed air is then used as impure reflux for the low pressure and/or high pressure column. The refrigeration for the top condenser of the high pressure column is provided by the vaporization of an intermediate liquid stream in the low pressure column.
U.S. Pat. No. 4,702,757 discloses a dual reboiler process in which a significant fraction of the feed air is partially condensed to provide reboil for the low pressure column bottom. The partially condensed air is then directly fed to the high pressure column. The refrigeration for the top condenser of the high pressure column is also provided by the vaporization of an intermediate liquid stream in the low pressure column.
U.S. Pat. No. 4,796,431 discloses a process with three reboilers located in the low pressure column. Also, U.S. Pat. No. 4,796,431 suggests that a fraction of the nitrogen removed from the top of the high pressure column is expanded to a medium pressure and then condensed against the vaporization of a fraction of the bottoms liquid from the lower column (crude liquid oxygen). This heat exchange will further reduce the irreversibilities in the upper column.
U.S. Pat. No. 4,936,099 also discloses a triple reboiler process. In this air separation process, the crude liquid oxygen bottoms from the bottom of the high pressure column is vaporized at a medium pressure against condensing nitrogen from the top of the high pressure column, and the resultant medium pressure oxygen-enrich air is then expanded through an expander into the low pressure column.
Unfortunately, the above cycles are only suitable for operation at low column operating pressures. As column pressure increases, the relative volatility between oxygen and nitrogen becomes smaller so more liquid nitrogen reflux is needed to achieve a reasonable recovery and substantial purity of the nitrogen product. The operating efficiency of the low pressure column of the above cycles starts to decline as the operating pressure increases beyond about 25 psia.
U.S. Pat. No. 4,224,045 discloses an integration of the conventional double column cycle air separation unit with a gas turbine. By simply taking a well known Linde double column system and increasing its pressure of operation, this patent is unable to fully exploit the opportunity presented by the product demand for both oxygen and nitrogen at high pressures.
Published European Patent Application No. 418,139 discloses the use of air as the heat transfer medium to avoid the direct heat link between the bottom end of the upper column and the top end of the lower column, which was claimed by U.S. Pat. No. 4,224,045 for its integration with a gas turbine. However, condensing and vaporizing the air not only increase the heat transfer area of the reboiler/condenser and the control cost, but also introduces extra inefficiencies due to the extra step of heat transfer, which makes its performance even worse than the Linde double column cycle.
U.S. patent application Ser. No. 07/700,021, issued as U.S. Pat. No. 5,165,245, discloses how the pressure energy contained in the pressurized nitrogen (or waste) streams can be efficiently utilized to make liquid nitrogen and/or liquid oxygen.